[PATCH] D85548: Allow fneg + strict_fadd -> strict_fsub in DAGCombiner

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


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Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:12824
+  // fold (strict_fadd A, (fneg B)) -> (strict_fsub A, B)
+  if (!LegalOperations || TLI.isOperationLegalOrCustom(ISD::STRICT_FSUB, VT))
+    if (SDValue NegN1 = TLI.getCheaperNegatedExpression(
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kpn wrote:
> arsenm wrote:
> > There is some strange legality predicate you are supposed to use for the strict cases, although I don't fully understand it
> Where is this predicate? Is there an existing case that can be modeled after?
> 
> My thinking is that this transform is probably safe because the FNeg will never trap and it will never round so it should therefore be safe to fold it into the add/sub. But I'm no expert on floating-point I admit.
This is totally valid. But for some reason, we have getStrictFPOperationAction which differs from checking the regular legalization action on the STRICT_ opcode


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  https://reviews.llvm.org/D85548/new/

https://reviews.llvm.org/D85548